I also have a pair of DeHavilland mono block amps with the same ground/float switches, and in the manual it states that leaving the switch in the "float" position may result in a "cleaner sound". At any rate, a fully functioning grounded power cord has done the trick...If the grounding scheme of the audio equipment is properly designed and executed there will be no need for a ground lift switch. The chassis will simply be properly grounded through the AC power cord's third prong and doing so will in no way affect the sound.
@jea48 has been correct in his assertions.